1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a semiconductor device having a capacitor.
2. Description of the Related Art
A conventional semiconductor device having an FET (field effect transistor), a semiconductor resistor element, and an MIM (Metal-Insulator-Metal) capacitor will be described below with reference to FIGS. 1A through 1H. First, Si is doped in the surface layer of a semi-insulating GaAs substrate 1 by ion implantation, and activated by annealing to form an active layer (n-type region) 2 and a semiconductor resistor element (n-type region) 3. Further, Si is doped at a high concentration in the opposite side areas of each of the active layer 2 and the semiconductor resistor layer 3, and activated by annealing to form contact layers (n+ regions) 4, 5 (FIG. 1A).
After the active layer 2, the semiconductor resistor layer 3, and the contact layers 4, 5 are formed, an SiNx layer 6 is formed on the whole surface of the GaAs substrate 1 (FIG. 1B). The SiNx film 6 is selectively removed by photolithography and an etching process to provide openings 7 in the areas thereof where the ohmic electrodes and the Schottky electrode of an FET and the ohmic electrodes of the semiconductor resistor element are to be formed (FIG. 1C). Succeedingly, by the processes such as photolithography, vapor deposition, lifting-off, and the like, ohmic electrodes 8, 9 made of a type of AuGe/Ni are formed on the respective contact layers 4, 5. After the ohmic electrodes 8, 9 are heat treated, a Schottky electrode 10 is formed on the active layer 2 (FIG. 1D). Thus, the FET 11 and a semiconductor resistor element (implant resistor) 12 are formed into the surface of the GaAs substrate 1.
Thereafter, a photoresist 13 is applied to the whole surface of the GaAs substrate 1. An opening is formed in the area of the photoresist 13 where the lower-side electrode of the capacitor is to be formed. An electrode material 14 comprising Ti/Pt/Au is sequentially vapor-deposited thereon (FIG. 2E). Succeedingly, the photoresist 13 is removed by a lifted-off method, and thereby, a lower layer electrode 15 made of Ti/Pt/Au is formed on the SiNx film 6 (FIG. 2F). Succeedingly, the whole surface of the GaAs substrate 1 including the lower layer electrode 15 is covered with an SiNx film 16. An opening 17 for wiring the lower layer electrode 15 is formed in the SiNx film 16 (FIG. 2G). Then, by processes such as photolithography, vapor deposition, lifting-off, and the like, an upper layer electrode 18 made of Ti/Au or Ti/Pt/Au is formed on the SiNx film 16 in the capacitor area (FIG. 2H). Thus, on the GaAs substrate 1, formed is an MIM capacitor 19 containing the upper layer electrode 18 and the lower layer electrode 15 opposite to each other through the SiNx film (dielectric layer) 16.
Ohmic electrodes for used with a field effect transistor (FET), a heterojunction bipolar transistor (HBT), a diode (Schottky barrier diode), a semiconductor resistor (an epitaxy resistor, an injected resistor), and the like, formed on a semiconductor substrate, are used in order to obtain an ohmic contact with the semiconductors (active layers). For this purpose, it is required to carry out the alloying by heat treatment, so-called alloying, by which a part of the ohmic electrodes are diffused in the semiconductors. By the alloying heat treatment, the ohmic electrodes have a larger surface roughness (morphology), as compared with those not heat treated for alloying. This will exert an influence over the breakdown voltage characteristics and the like. Accordingly, it has been thought that the ohmic electrodes are unsuitable for ordinary wiring. Particularly, the ohmic electrodes have never been formed on insulating films, since their purpose is to provide an ohmic contact with the semiconductors.
For a capacitor, its upper layer and lower layer electrodes are formed on an insulation film. Accordingly, the upper layer and lower layer electrodes, which are required to have high breakdown voltage characteristics, were not formed with the same metallic material as that of the ohmic electrodes, but with a metallic material (for example, Ti/Au or Ti/Pt/Au) different from that of the ohmic electrodes.
Thus, in the conventional semiconductor device, the upper layer and lower layer electrodes of the capacitor were formed with a metallic material different from that of an element such as FET, a diode, or the like and by a process different from that for the elements. This complicated the production process for the semiconductor device, and affected the reliability of its electric characteristics.
In view of the foregoing, the present invention has been devised, the object of which is to simplify the process of producing a semiconductor device, and to improve the reliability of its electric characteristics by formation of the upper layer electrode or the lower layer electrode of the capacitor with the same metal material as that of the ohmic electrode(s).
A semiconductor device according to the present invention includes a capacitor having a pair of electrodes opposite to each other through a dielectric layer, and an element other than a capacitor, both of which are formed on a semiconductor substrate, wherein an ohmic electrode of the element and one of the electrodes of the capacitor are formed with the same metallic material.
According to the present invention, one of the electrodes of the capacitor is formed with the same metallic material as that of the ohmic electrode. Therefore, the one of the electrodes of the capacitor, together with the ohmic electrode, can be simultaneously formed by the same process. Thus, the process parameters (photolithographic conditions, vapor deposition conditions, and the like) to be controlled in the process of forming the electrodes can be reduced. That is, according to the present invention, the number of processes and the control parameters required for the formation of the electrodes of an element such as an FET or the like and the capacitor can be reduced, the production process for the semiconductor device can be simplified, and the production cost can be reduced. In addition, variations in the electrical characteristics of the semiconductor device can be decreased, and the reliability of the electrical characteristics can be enhanced, due to the simplified production process and the reduced number of control parameters.
Preferably, the electrode of the capacitor made of the same metallic material as that of the ohmic electrode is formed on the upper side of an insulating film formed above the semiconductor substrate. However, the electrode of the capacitor may be formed directly on the semiconductor substrate.
According to the knowledge obtained experimentally by the inventor of the present invention, the surface roughness of the ohmic electrode, which was formed on an insulating film (for example, an SiNx film, an SiO2 film, an SiON film, or the like), suffered substantially no changes by the alloying heat treatment. In addition, it has been revealed that the adhesion of the ohmic electrode to the insulating film becomes higher, as compared with that before the heat treatment.
Thus, owing to the formation of the electrode of the capacitor on the insulating film, the electrode of the capacitor, though it is formed with the same metallic material as the ohmic electrode, can be rendered a smooth-surface without the deterioration of the surface roughness, which will caused by the heat treatment, and the reliability of the capacitor characteristics such as the withstand voltage characteristics and the like can be sufficiently enhanced.
Further, the lower layer electrode was formed with the same metallic material as the ohmic electrode on the semiconductor substrate, not on the insulating film, and the capacitor characteristics were investigated. As a result, the capacitor characteristics comparable to those of a conventional MIM capacitor could be obtained. Particularly, the withstand voltage characteristics were not different, though they were conventionally thought to reflect on the difference between the surface roughnesses.
Preferably, the ohmic electrode and the electrode of the capacitor made of the same metallic material are heat treated at a temperature of from 350xc2x0 C. to 450xc2x0 C. This temperature condition is ordinarily used for the heat treatment of the ohmic electrode. This means that the conventional temperature condition for the ohmic electrode is neither changed nor complicated.
The electrode of the capacitor made of the same material as the ohmic electrode may contain Au, Ge, and Ni. It is not necessary to use an especial metallic material for the ohmic electrode.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.